1. Field of the Invention
The present invention relates to new powder coating compositions, wherein the binder is based on a mixture of copolymers containing hydroxyl and carboxylic anhydride groups with polyepoxide compounds.
2. Description of the Prior Art
The use of compositions containing polyacrylates having at least two carboxyl groups per molecule and polyepoxide compounds, which may be based on acrylates, is known.
Japanese patent application 73-29 319 (publication number: 49 116 134; C. A. 82(24): 157 882 e) describes heat-curing coating powders containing A) a solid acrylate resin prepared by reacting a hydroxyl-functional copolymer with a polycarboxylic acid or anhydride and B) an epoxy-functional copolymer. The two polymers are mixed in the form of hot melts (100.degree. C.) and, after cooling, the resulting mixture is ground. The coatings are applied to metal surfaces and cured at temperatures of about 200.degree. C.
DE-OS 2,347,680 describes heat-curing powder coatings containing A) 50 to 97% of an epoxy-functional copolymer having a molecular weight of 3,000 to 15,000 and a softening point of 80.degree. to 150.degree. C. and B) 3 to 50% of a carboxyl-functional curing agent having a softening point of at least 50.degree. C.; the curing agent may also be a carboxyl-functional copolymer.
Japanese patent application 75-146 705 (publication number: 52 069 936; C.A. 88(12): 75 374 a) describes heat-curing powder coatings based on glycidyl (meth)acrylate o copolymers containing A) 100 parts of a copolymer (Tg 30 to 100.degree. C., molecular weight 3,000 to 70,000) containing 10 to 50% by weight glycidyl (meth)acrylate, 30 to 85% by weight C.sub.1-12 alkyl acrylate and 0 to 60% by weight of another comonomer, B) 0.1 to 30 parts of a flow-regulating copolymer (Tg&gt;50.degree. C., molecular weight &gt;1,000) containing 30 to 85% by weight of an alkyl (meth)acrylate, 1 to 50% by weight of a monomer containing a glycidyl, carboxyl, hydroxyl or amide group and 0 to 69% by weight of another monomer and C) 3 to 55 parts of an aliphatic dicarboxylic acid containing 4 to 20 carbon atoms. The paints show good adhesion to metals without preliminary surface treatment.
U.S. Pat. Nos. 3,991,132 and 3,991,133 describe heat-curing coating powders containing A) an epoxy-functional copolymer prepared from of 5 to 20% glycidyl methacrylate and 80 to 95% of other comonomers (Tg 40.degree. to 90.degree. C., molecular weight 1,500 to 15,000) and B) an anhydride crosslinking agent which is either a dicarboxylic anhydride or a dicarboxylic anhydride homopolymer, provided that the copolymer is difunctional. In addition to 70 to 93% of other comonomers such as methyl or butyl methacrylate, styrene and vinyl toluene, U.S. Pat. No. 3,991,132 discloses that the anhydride crosslinking agent is prepared from at least 2 to 10% of an unsaturated acid amide, while U.S. Pat. No. 3,991,133 discloses that the anhydride crosslinking agent is prepared from at least 2 to 10% of a C.sub.5-7 hydroxyalkyl (meth)acrylate. Further, the anhydride curing agent is replaced by a hydroxycarboxylic acid in such a quantity that, for every functional group of the copolymer, there are 0.1 to 0.4 carboxyl groups of the hydroxycarboxylic acid.
It has now surprisingly been found that copolymers of olefinically unsaturated compounds containing both incorporated hydroxyl groups and incorporated intramolecular carboxylic anhydride groups in the same molecule, in combination with polyepoxides, represent excellent binders for powder coatings, provided that they possess the necessary softening point or softening range. The particular advantage of such systems is that component A) initially contains no free carboxyl groups which are necessary for crosslinking with the epoxide groups of component B). Instead these carboxyl groups are only formed from the reaction of the carboxylic anhydride groups with hydroxyl groups by the action of heat. Accordingly, these binder composition show considerably better storage stability at room temperature than comparable prior art systems which contain free carboxyl groups. Another advantage of the binder compositions according to the invention is that the hardening reaction is based on a so-called "double-cure" mechanism, i.e., the hydroxyl groups of component A) initially react with the anhydride groups of component A) under the effect of heat or catalysts with opening of the anhydride ring and formation of carboxyl groups. This reaction is itself a crosslinking reaction. In addition, the newly formed carboxyl groups can react with the epoxide groups of component B) which results in additional crosslinking.
By virtue of the high crosslink densities which may be obtained, coatings characterized by very high resistance to chemicals and solvents are obtained from the binder compositions according to the invention.
Accordingly, the binder combinations according to the invention show excellent stability in storage at room temperature or moderately elevated temperatures below their softening range and, on the other hand, provide for the production of coatings having excellent performance properties.